This invention relates to a magnetic recording correction circuit for a video tape recorder, and more particularly, to a correction circuit capable of reducing an interference wave due to the third-degree inter-modulation (cross modulation) distortion accompanying recording/playback of an information signal.
FIGS. 1A and 1B show the frequency allocation by the prior art. With reference to these diagrams on occasion, the prior art will now be described.
In the Video Home System (VHS) standard which is one system of a home Video Tape Recorder (VTR), a frequency modulated (FM) luminance signal and a low-frequency band chrominance signal are frequency-multiplexed. The signal thus multiplexed is recorded on the surface layer portion of a magnetic tape by means of a first magnetic head. The above-mentioned FM luminance signal is provided by frequency-modulating a carrier with a luminance signal, and the above-mentioned low frequency band chrominance signal is provided by converting a chrominance signal to signal components in a low frequency band. In the above-described VHS standard, a first frequency-modulated (FM) audio signal and a second frequency-modulated (FM) audio signal are frequency-multiplexed. The signal thus multiplexed is recorded at the deep layer portion of the magnetic tape by means of a second magnetic head. The above-mentioned first and second FM audio signals are provided by frequency-modulating carriers with audio signals of two channels, respectively. Further, there is employed a configuration such that the second magnetic head is precedent to the first magnetic head, and is adapted to scan the same track as that of the first magnetic head with these magnetic heads having azimuth angles different from each other.
Usually, each of the video signal (FM luminance signal and low-frequency band chrominance signal) and the multiplexed FM audio signal is recorded and reproduced by a pair of magnetic heads having mutually opposite azimuth angles alternately track by track. But it is not a substantial matter for the present invention, and only one head for each is illustrated or explained in this specification.
FIG. 1A shows the frequency allocation of a signal recorded by the first magnetic head wherein reference numeral 1 represents an occupied frequency band of the low frequency band chrominance signal, reference numeral 2 represents an occupied frequency band of a FM luminance signal, and reference numeral 1a represents a carrier frequency (0.629 MHz) of the low frequency band chrominance signal. Further, FIG. 1B shows the frequency allocation of audio signals recorded by the second magnetic head wherein reference numeral 3 represents an occupied frequency band of a first FM audio signal, reference numeral 4 represents an occupied frequency band of a second audio signal, fa represents a carrier frequency (1.3 MHz) of the first FM audio signal, and fb represents a carrier frequency (1.7 MHz) of the second FM audio signal.
Meanwhile, it is known in the magnetic recording/reproduction that when two signals having carrier frequencies different from each other are simultaneously recorded and reproduced, a third-degree inter-modulation distortion would take place by the non-linearlity occuring in the process of the magnetic recording/reproduction. On the basis of such a characteristic, between the first FM audio signal (1.3 MHz) and the second FM audio signal (1.7 MHz), there take place six third-degree inter-modulation distortions of "3.9 (=1.3.times.3) MHz", "0.9 (=1.3.times.2-1.7) MHz", "4.3 (=1.3.times.2+1.7) MHz", "2.1 (=1.7.times.2-1.3) MHz", "4.7 (=1.7.times.2+1.3) MHz", and "5.1 (=1.7.times.3) MHz". Such third-degree inter-modulation distortions are reproduced by the first magnetic head, but most thereof do not cause interference owing to the azimuth angle loss.
However, the lowest frequency component "0.9 (=1.3.times.2-1.7) MHz" is of problem because its azimuth angle loss is small. Namely, as shown in FIG. 1A, since the occupied frequency band 1 of the low frequency band chrominance signal exists from "0.1 MHz" to "1.1 MHz", the frequency component of "0.9 MHz" becomes an interference wave with respect to the low frequency band chrominance signal. As a result, there is the problem that in the case where such a signal is demodulated, an oblique stripe pattern appears on the entirety of a picture.
With a view of avoiding such a problem, in the prior art, an approach was employed to shift recording currents of the first and second FM audio signals recorded by the second magnetic head from an optimum value to thereby lower the recording level of the frequency component of "0.9 MHz" which is the above-mentioned third-degree inter-modulation distortion to hold down the level of an interference wave reproduced by the first magnetic head to a lower level, thus to cope with this problem.
In accordance with the above-described prior art, since the recording current of the second magnetic head is shifted from the optimum value, the regenerative level of the FM audio signal is lowered. For this reason, there was the problem that S/N of the reproduced audio signal is lowered.
Further, even if the recording current of the second magnetic head is shifted from the optimum value, because the recording efficiency depends upon the depth of the gap of the magnetic head, the recording efficiency is increased by abrasion or wear of the second magnetic head. As a result, reduction in the recording level of the lowest frequency of the third-degree inter-modulation distortion by reduction of the recording current of the second magnetic head is canceled. Thus, there results the state equivalent to the state where that level is not reduced. For this reason, there also was the problem that an oblique stripe pattern might appear on a picture.